1. Field of the Invention
The present invention relates to a method and an apparatus for controlling discharged-air temperature for a vehicular air-conditioning system including an upper outlet disposed at an upper position for discharging air into a passenger's compartment and a lower outlet disposed at a lower position for discharging air into the passenger's compartment.
2. Description of the Related Art
In general, a variety of systems are adopted in order to perform air-conditioning in a passenger's compartment of a vehicle. Those widely used include, for example, a reheat system in which all of cold air, which has been dehumidified after passing through an evaporator, is allowed to pass through a heater core to heat the cold air so that the temperature-controlled and/or humidity-controlled air is discharged from respective outlets into the passenger's compartment, and an air-mixing system in which air, which has passed through an evaporator, is divided by an air-mixing damper into air to make a passage through a heater core and air to make a detour to avoid the heater core, followed by mixing hot air and cold air at a position downstream of the heater core to obtain mixed air which is discharged from respective outlets into the passenger's compartment.
The outlet mode of the vehicular air-conditioning system for discharging air into the passenger's compartment includes a "DEF" (defrosting) mode, a "FOOT/DEF" mode, a "FOOT" mode, a "B/L" (bi-level) mode, and a "VENT" mode (or "FACE" mode).
In the case of such a vehicular air-conditioning system, it is required to heat the passenger's compartment such that the passenger feels no burning sensation at face or head while maintaining a heating sensation at foot of the passenger. For this purpose, for example, the air-mixing control is performed, in which a predetermined temperature difference is given between temperatures of airs discharged from upper and lower outlets, i.e., airs discharged from the face outlet and the foot outlet during the "FOOT/DEF" mode and the "B/L" mode.
However, in the case of a heat pump air-conditioning system provided with a heat pump-type cooling cycle arranged with a condenser for heating the passenger's compartment by condensing a coolant in a duct to radiate heat, it is impossible to give the temperature difference between the discharged-air temperature at the face outlet and the discharged-air temperature at the foot outlet. Therefore, it is conceived for the heat pump air-conditioning system to combine the reheat type temperature control and the air-mixing type temperature control. As for such a combination, for example, a vehicular air-conditioning system is known as disclosed in Japanese Laid-Open Patent Publication No. 8-67134.
An object of the conventional air-conditioning system is to avoid excessive increase in motive power and excessive increase in electric power consumption of an electric compressor used for the heat pump air-conditioning system. In order to achieve the object, the conventional air-conditioning system comprises a discharged-air temperature control unit based on the use of both of the reheat type temperature control effected by controlling a compressor and the air-mixing type temperature control effected by controlling an air-mixing damper, for controlling the temperature of air discharged from the face outlet and the foot outlet. The discharged-air temperature control unit performs the air-mixing type temperature control only when the electric power consumed by the compressor is not more than a reference electric power. The discharged-air temperature control unit performs the air-mixing type temperature control only when the calculated target discharged-air temperature is not higher than a reference temperature, for example, 70.degree. C.
However, in the case of the conventional air-conditioning system, the air-mixing type temperature control is not performed when the target discharged-air temperature is higher than the reference discharged-air temperature. Therefore, it is feared that high temperature air is discharged especially from the face outlet.
Further, the conventional air-conditioning system performs the air-mixing type temperature control in order to give the difference in temperature between the upper and lower outlets. When the outlet mode is the "VENT" mode, the ordinary reheat type temperature control is performed. Therefore, the following problem is pointed out. That is, when the operation mode is changed during the heating operation from the "HEAT" mode to the "VENT" mode, then air at a considerably high temperature is discharged against the face of the passenger, and the passenger feels an uncomfortable sensation. Especially, when it is intended to suddenly lower the discharged-air temperature, it is feared that air at a high temperature may be discharged from the face outlet against the face of the passenger.